Laboratory Assessment of Nostoc 9v (Cyanobacteria) Effects on N2 Fixation and Chemical Fertility of Degraded African Soils

Abstract

The potential of Nostoc 9v for improving the nitrogen (N)2–fixing capacity and nutrient status of semi-arid soils from Tanzania, Zimbabwe, and South Africa was studied in a laboratory experiment. Nostoc 9v was inoculated on nonsterilized and sterilized soils. Inoculum rates were 2.5 mg dry biomass g21 soil and 5mg dry biomass g21 soil. The soils were incubated for 3 months at 27 uC under 22Wm2 illumination with a photoperiod of 16 h light and 8 h dark. The moisture was maintained at 60% of field capacity. In all soils, Nostoc 9v proliferated and colonized the soil surfaces very quickly and was tolerant to acidity and low nutrient availability. Cyanobacteria promoted soil N2 fixation and had a pronounced effect on total soil organic carbon (SOC), which increased by 30–100%. Total N also increased, but the enrichment was, in most soils, comparatively lower than for carbon (C). Nitrate and ammonium concentrations, in contrast, decreased in all the soils studied. Increases in the concentration of available macronutrients were produced in most soils and treatments, ranging from 3 to 20mg phosphorus (P) kg21 soil, from 5 to 58mg potassium (K) kg21 soil, from 4 to 285mg calcium (Ca) kg21, and from 12 to 90 mg magnesium (Mg) kg21 soil. Positive effects on the levels of available manganese (Mn) and zinc (Zn) were also observed